RACH is a channel for acquiring initial uplink synchronization. If a terminal (i.e., UE) is firstly powered on, or the terminal (UE) is switched to an active status after it has been in an idle mode for a long period of time, uplink synchronization should be re-established. The RACH is generally adapted to re-establish the uplink synchronization, and need not establish time synchronization or frequency synchronization.
The RACH basically supports a plurality of users (hereinafter referred to as a multi-user). Each user equipment (UE) transmits a specific preamble sequence when accessing the RACH.
If a base station (hereinafter referred to as a Node-B) recognizes the preamble sequence and transmits the recognized preamble sequence, the user equipment (UE) updates its own time synchronization information using the aforementioned preamble sequence information. In this case, if the Node-B transmits frequency synchronization information along with the time synchronization information, this frequency synchronization information can also be used to correct the user equipment (UE).
FIG. 1 is a basic RACH structure based on the presence or absence of a cyclic prefix (CP).
Referring to FIG. 1, the RACH can be classified into two RACH types 101 and 102. The first RACH 101 uses the cyclic prefix (CP), and the second RACH 102 does not use the cyclic prefix (CP).
If the CP is applied to the RACH as shown in the first RACH 101, it can maintain the orthogonality between channels by reducing interference between channels, however it slightly reduces the sequence length. If the sequence length becomes shorter, correlation characteristics become deteriorated, so that it may have a negative influence upon the detection performance.
In other words, if the RACH does not use the CP as shown in the second RACH 102, the preamble length becomes longer, but orthogonality between sequences cannot be maintained when the second RACH 102 searches for the preamble in the frequency area.
In the meantime, since the RACH preamble is a signal transmitted before a closed loop between the UE and the Node-B is not formed, the RACH preamble is generated by the user equipment (UE) and is then transmitted, so that it is vulnerable to the frequency offset. If the Node-B receives the RACH affected by the aforementioned frequency offset, a false alarm rate of the Node-B may increase in the Node-B or the detection probability may be decreased in the same Node-B.
Therefore, in order to cope with the negative influence of the frequency offset in the RACH transmission, many developers are conducting intensive research into a method for establishing the RACH to cope with the frequency offset and a method for allowing the user equipment (UE) to transmit the RACH according to the aforementioned RACH setup information.
Presently, according to the 3GPP LTE, the RACH needs to be operated in a large-sized cell without any problem, so that an improved RACH structure for satisfying this requirement must be designed.